Ion beam irradiation in La2Zr2O7-Ce2Zr2O7 pyrochlore

Generally, zirconate pyrochlores do not experience a radiation-induced transformation from the crystalline-to-amorphous state, but rather disorder to a defect fluorite structure-type. Thus Gd2Zr2O7 has been proposed as a nuclear waste form for the immobilization of plutonium because of its radiation "stability". In contrast, La2Zr2O7 can be amorphized by a 1.5 MeV Xe+ ion irradiation (similar to5.5 dpa at room temperature), and the critical amorphization temperature is low (similar to310 K). In this study we present data on ion beam irradiations of compositions in the solid solution: (La1-xCex)(2)Zr2O7 (x = 0, 0.1, 0.2, 1). Ce is used as an analogue element for Pu because of similarities in charge and size. La2Zr2O7 can be amorphized by a 1.0 MeV Kr+ ion irradiation at 25 and 293 K at doses of similar to1.19 and similar to3.42 dpa, respectively, confirming that La2Zr2O7 is Susceptible to ion irradiation-induced amorphization. With the addition of 10 mol% Ce in lanthanum zirconate pyrochlore structure, no ion irradiation-induced amorphization has been observed at room temperature. The critical amorphization dose for (La0.9Ce0.1)(2)Zr2O7 at 25 K is similar to3.55 dpa, and with increasing Ce-content, a higher dose (similar to5.20 dpa) is required to fully amorphize (La0.8Ce0.2)(2)Zr2O7 at 25 K. No amorphization occurred for Ce2Zr2O7 at 25 K at a. dose of similar to7 dpa. These results suggest that the addition of Ce into the La2Zr2O7 structure increases the stability of the La2Zr2O7 waste form in a radiation environment, which may be attributed to the decreasing average radius of cations in the A-site, resulting from the smaller ionic radii of Ce3+ (0.114 nm) and Ce4+ (0.097 nm) as compared to La3+ (0.116 nm). An ion beam-induced anion-disordered pyrochlore was observed prior to the final transformation to a disordered fluorite structure. The local structural evolution upon ion irradiation was also investigated for Ce-doped La2Zr2O7 pyrochlores with different average A-site cation sizes and valence states using electron energy-loss spectrometry (EELS). (C) 2004 Elsevier B.V. All rights reserved.